17. Cell Wall of Gram negative bacteria. The Outer Membrane of Gram-negative Bacteria

Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain(commonly safranin) is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color. This is because of the existence of an outer membrane preventing the penetration of the stain. The test itself is useful in classifying two distinct types of bacteria based on the structural differences of their bacterial cell walls. Gram-positive bacteria will retain the crystal violet dye when washed in a decolorizing solution. Compared with Gram-positive bacteria, Gram-negative bacteria are more resistant against antibodies, because of their impenetrable wall.The pathogenic capability of Gram-negative bacteria is often associated with certain components of Gram-negative cell envelope, in particular, thelipopolysaccharide layer (also known as LPS or endotoxin layer). In humans, LPS triggers an innate immune response characterized by cytokineproduction and immune system activation. Inflammation is a common result of cytokine (from the Greek cyto, cell and kinesis, movement) production, which can also produce host toxicity.

Characteristics

The following characteristics are displayed for Gram-negative bacteria:

1)Cytoplasmic membrane

2)Thin peptidoglycan layer (which is much thicker in Gram-positive bacteria)

3)Outer membrane containing lipopolysaccharide (LPS, which consists of lipid A, core polysaccharide, and O antigen) in its outer leaflet and phospholipids in the inner leaflet

4)Porins exist in the outer membrane, which act like pores for particular molecules

5)There is a space between the layers of peptidoglycan and the secondary cell membrane called the periplasmic space

6)The S-layer is directly attached to the outer membrane, rather than the peptidoglycan

7)If present, flagella have four supporting rings instead of two

8)No teichoic acids or lipoteichoic acids are present

9)Lipoproteins are attached to the polysaccharide backbone.

10)Most of them contain Braun's lipoprotein, which serves as a link between the outer membrane and the peptidoglycan chain by a covalent bond

11)Most do not sporulate (Coxiella burnetii, which produces spore-like structures, is a notable 

The proteobacteria are a major group of Gram-negative bacteria, including Escherichia coli (E. coli), Salmonella, Shigella, and other Enterobacteriaceae, Pseudomonas, Moraxella, Helicobacter,Stenotrophomonas, Bdellovibrio, acetic acid bacteria, Legionella and numerous others. Other notable groups of Gram-negative bacteria include the cyanobacteria, spirochaetes, green sulfur andgreen non-sulfur bacteria.Medically relevant Gram-negative cocci include three organisms, which cause a sexually transmitted disease (Neisseria gonorrhoeae), a meningitis (Neisseria meningitidis), and respiratory symptoms (Moraxella catarrhalis).Medically relevant Gram-negative bacilli include a multitude of species. Some of them primarily cause respiratory problems (Hemophilus influenzae, Klebsiella pneumoniae, Legionella pneumophila, Pseudomonas aeruginosa), primarily urinary problems (Escherichia coli, Proteus mirabilis, Enterobacter cloacae, Serratia marcescens), and primarily gastrointestinal problems (Helicobacter pylori, Salmonella enteritidis, Salmonella typhi).Gram-negative bacteria associated with nosocomial infections include Acinetobacter baumannii, which cause bacteremia, secondary meningitis, and ventilator-associated pneumonia in intensive-care units of hospital establishments.